Articulated forklift truck

ABSTRACT

An articulated forklift truck for use in narrow aisles comprises a front chassis section ( 10 ) having a first pair of ground-engaging wheels ( 20 ) and a rear chassis section ( 12 ) having a second pair of ground-engaging wheels ( 22 ). The front chassis section ( 10 ) carries a mast ( 16 ) and lift forks ( 18 ) and is pivoted to the rear chassis section ( 12 ) about a substantially vertical axis ( 14 ) to allow the truck to be steered. Both front wheels ( 20 ) but only one of the rear wheels ( 22 ) are driven by electric of hydrostatic motors ( 24, 26 ).

FIELD OF THE INVENTION

This invention relates to an articulated forklift truck, especially ofthe kind designed for use in narrow aisles.

BACKGROUND TO THE INVENTION

A typical narrow-aisle articulated forklift truck comprises front andrear chassis sections each having a pair of wheels on a respectivecommon axis. The rear wheels are driven, while the front wheels arenon-driven. The chassis sections are pivoted together about a verticalaxis so that the front chassis section can be turned at an angle at orapproaching 90° relative to the rear chassis section to allow the truckto insert loads into, and remove loads from, the face of the aisle, i.e.normal to the length of the aisle.

Clearly, as the steering angle increases the inner rear wheel, i.e. thewheel on the inside of the path taken by the truck, will turn moreslowly than the outer rear wheel, and in fact may stop altogether orreverse if the axis of the front wheels coincides with or lies insidethe ground contact point of the inner rear wheel at large steeringangles. In conventional narrow-aisle articulated forklift trucks thisleads to tyre wear and difficulties in manoeuvring and controlling thetruck. At large steering angles, too, the rear driving wheels will tendto drive the forklift straight ahead (this is known as “dozing on”),with the front wheels skidding sideways.

It is an object of this invention to provide a construction of forklifttruck in which this problem is avoided or mitigated.

SUMMARY OF THE INVENTION

Accordingly to the present invention there is provided an articulatedforklift truck comprising a rear chassis section having a rear pair ofground-engaging wheels on a first substantially common axis, a frontchassis section pivoted to the rear chassis section about asubstantially vertical axis and having a front pair of ground-engagingwheels on a second substantially common axis, the front chassis sectioncarrying a mast and lift forks, and steering means for turning the frontchassis section relative to the rear chassis section, the truck furtherincluding means for driving both front wheels and one only of the rearwheels.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a schematic plan view of a narrow-aisle articulated forklifttruck according to the invention in the straight-ahead position;

FIG. 2 is a schematic diagram of the hydraulic circuit used to drive thehydrostatic motors associated with the wheels of the truck of FIG. 1;

FIG. 3 is a plan view of the truck of FIG. 1 in the act of manoeuvringgoods into a loading bay; and

FIG. 4 is a perspective view of the truck of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, an articulated forklift truck 8 comprisesfront and rear chassis sections 10, 12 respectively. The rear chassissection 12 is pivotable about a vertical shaft 14 fixed to the frontchassis section 10. The truck also includes a conventional poweredsteering mechanism, not shown, which allows the vehicle operator, who isseated on the rear chassis section 12, to steer the truck by turning thefront chassis section 10 left or right through up to 90° relative to therear chassis section 12.

The front chassis section 10 carries a conventional mast 16 and liftforks 18. The front chassis section 10 has a first pair ofground-engaging wheels 20 on a common axis, and the rear chassis section12 has a second pair of ground-engaging wheels 22 on a common axis. Theaxis of the front wheels 20 is tiltable in a vertical plane containingsaid axis to accommodate irregularities in the ground over which thetruck travels.

Each front wheel 20 is individually driven by a respective hydrostaticmotor 24, but one only of the rear wheels 20 is individually driven by ahydrostatic motor 26 (the other rear wheel 20 is allowed to freewheel).As is well known, each motor 24, 26 has a number of pistons whichoperates on a cam ring to drive the respective wheel 20 or 22. Avariable displacement pump 28 powered by an internal combustion engine30 supplies hydraulic fluid to the motors 24 and 26 in known manner.

Referring to FIG. 2, under the control of the vehicle operator the pump28 can be selectively operated to supply hydraulic fluid under pressureeither via line A to drive all three motors, and hence all three drivenwheels, simultaneously in a forward direction, or via line B to driveall three motors, and hence all three driven wheels, simultaneously in areverse direction.

To manoeuvre a load 32, FIG. 3, into the loading bay 34, it is necessaryto manoeuvre the forklift truck 8 such that the axis of the front wheels20 is substantially at 90° to the axis of the rear wheels 22 in order toposition the load 32 in the loading bay 34 since the aisle 36 isrelatively narrow when compared with the width of the body 12.

The displacement and torque of the wheel motors 24, 26 is selected suchthat, when the truck is loading the goods into the loading bay as shownin FIG. 3, the supply of hydraulic fluid under pressure to line A issufficient to rotate the front wheels 20 in a forward direction to movethe load towards and into the loading bay 34. Because of the geometry ofthe truck one of the rear wheels, i.e. that on the inside of the pathtaken by the truck, may actually rotate in the reverse direction and ifit is the driven wheel, such rotation will be against the force of theassociated motor 26 tending to drive it forwardly.

The operation of the front and rear motors 24, 26 prevents the frontwheels 20 from being forced sideways in the direction of arrow A alongthe aisle and increases the manoeuvrability of the forklift truckenabling goods to be loaded and unloaded in confined spaces such asnarrow aisles or warehouses.

The front wheels 20 pull and steer the machine in the requireddirection. As hydraulic oil takes the path of least resistance it ismuch easier to drive the front wheels in the required direction than theback wheel to doze the front wheels forwards.

Variations of the above embodiment are possible. For example, althoughthe embodiment uses hydrostatic motors, these could alternatively beelectric motors. Also, irrespective of whether hydrostatic or electricmotors are used, a single motor 24 could be used to drive the frontwheels 20. This could be achieved by, for example, fitting the frontwheels 20 at opposite ends of a common axle. The axle may be fitted witha differential and is driven by a motor. Then, a second motor 26 wouldbe used to individually drive the rear driven wheel 22.

It may also be advantageous to remove the drive power to the rear motor26 when the angle between the front and rear chassis sections 10, 12exceeds a predetermined angle relative to the straight ahead position,for example, when the front section 10 turns through more than 45°relative to the rear section 12. This could be achieved by fixing aswitch (not shown) to the rear chassis section, the switch beingoperated by a member fixed to the front chassis section when the latterturns more than the predetermined angle. Operating the switch wouldremove electric or hydraulic power, as the case may be, to the rearmotor 26. This feature could be used irrespective of whether the motorsare hydrostatic or electric, or whether the front wheels 20 are drivenin common by a single motor or individually by separate motors.

The invention is not limited to the embodiment described herein and maybe modified or varied without departing from the scope of the invention.

1. An articulated forklift truck comprising a rear chassis sectionhaving a rear pair of ground-engaging wheels on a first substantiallycommon axis, a front chassis section pivoted to the rear chassis sectionabout a substantially vertical axis and having a front pair ofground-engaging wheels on a second substantially common axis, the frontchassis section carrying a mast and lift forks, and steering means forturning the front chassis section relative to the rear chassis section,the truck further including means for driving both front wheels and oneonly of the rear wheels.
 2. A forklift truck according to claim 1,wherein the driving means comprises a respective motor for individuallydriving each of the three driven wheels.
 3. A forklift truck accordingto claim 1, wherein the driving means comprises a single motor fordriving the front wheels and a further motor for individually drivingthe rear driven wheel.
 4. A forklift truck according to claim 2, whereineach motor is a hydrostatic motor.
 5. A forklift truck according toclaim 2, wherein each motor is an electric motor.
 6. A forklift truckaccording to claim 2, wherein the driving means is selectively operableto drive all three driven wheels simultaneously in a forward directionor simultaneously in a reverse direction at the same time.
 7. A forklifttruck according to claim 6, including means to disengage the drive tothe rear motor when the angle between the front and rear chassissections exceeds a predetermined angle.
 8. A forklift truck according toclaim 3, wherein each motor is a hydrostatic motor.
 9. A forklift truckaccording to claim 3, wherein each motor is an electric motor.